The compact disc (CD) was invented in the 1980s to allow for an all-digital recording of audio signals. The optical pick-up unit (OPU) for audio-CD and/or CD-ROM uses a near-infrared (NIR) (e.g., 780 nm, 785 nm, 790 nm) semiconductor laser to read-out the encoded digital information. The numerical aperture (NA) of the objective lens is about 0.45, allowing a pit (one unit of encoding on disc) measuring about 100 nm deep, 500 nm wide and 850 nm to 3500 nm long depending on the radial distance from the disc center.
The first commercial digital versatile disc (DVD) appeared in the 1990s, with crucial optical design changes to allow for a physical recording density increase of about 3.5 times CDs. The gain in physical density was made possible by employing a shorter wavelength semiconductor (SC) laser (e.g., 650 nm, 660 nm red band, etc. compared to 780 nm near-IR band (NIR) in audio-CD) and a larger NA lens (e.g., 0.6 NA requiring a 0.6 mm thick DVD disc). A backward compatible DVD/CD optical pick-up unit employs two laser sources, either packaged as a single component or discretely, that have their read beams coupled by polarization beam combiners (PBCs) and/or dichroic beam combiners (DBCs).
Successors to the DVD media format ranges from Blu-ray Disc (BD) to high density HD-DVD. In these systems, the read/write SC laser wavelength is further decreased to about 405˜410 nm blue-violet band and the NA is increased to about 0.85. In BD or HD-DVD backward compatible DVD/CD systems, a third wavelength laser (e.g., co-packaged or discrete with respect to the first two lasers) is required to support all three disc media formats.
Referring to FIG. 1, there is shown one example of a prior art 3-wavelength HD-DVD/DVD/CD optical pick-up unit (OPU). The OPU 100 includes an array of semiconductor laser sources 110 (i.e., shown as three discrete laser diodes (LD) including a first LD 111 at λ=780 nm, a second LD 112 at λ=660 nm, and a third LD 113 at λ=405 nm), the output of which are spatially multiplexed by an array of polarization beam combiner cubes (PBC) 130, is collimated by a lens system 160 and is folded by a leaky mirror 140 before being imaged (focused) onto a single “pit” area on the rotating disc media 150 via an objective lens 161. The leaky mirror 140 allows for a small fraction (e.g., 5%) of the incident beam energy to be tapped off and focused onto a monitor photodiode (PD) 175 via another lens 165.
The output from the array of LD sources 110 is substantially linearly polarized (e.g., ‘S’ polarized with respect to the PBC hypotenuse surface). Prior to reaching the array of PBC cubes 130, these linearly polarized beams are transmitted through an array of low-specification polarizers 120, which protect the LD sources from unwanted feedback (e.g., “P” polarized light). Conventionally, the protection filters 120 are simple dichroic absorptive polarizers with a 10:1 polarization extinction ratio.
The main ray from each of the LD sources 110 is directed along the common path 180 towards the disc media 150. Prior to reaching the quarter-waveplate (QWP) 145, the light is substantially linearly polarized. After passing through the QWP 145, the linearly polarized (LP) light is transformed into circularly polarized (CP) light. The handedness of the CP light is dependent on the optic axis orientation of the QWP (for a given S- or P-polarized input). In the example shown, with ‘S’ polarization input to the QWP, if the slow-axis of the QWP is aligned at 45° counter clockwise (CCW), with respect to the p-plane of the PBC, a left-handed circularly (LHC) polarized results at the exit of the QWP (LHC, having a Jones vector [1 j]T/√2 and with the assumption of intuitive RH-XYZ coordinate system while looking at the beam coming to the observer).
In a pre-recorded CD and DVD disc, where there is a physical indentation of a recorded pit, the optical path length difference between a pit and its surrounding “land”, at ⅙ to ¼ wave, provides at least partial destructive interference and reduces the light detected by the main photodiode 170 positioned at the second port of the PBC cube array 130. On the other hand, the absence of a pit causes the change of the CP handedness, at substantially the same light power in its return towards the PBC cube array 130. The light has effectively been transformed by the QWP in double-passing to convert the initially S-polarized light to P-polarized light on its return to the PBC array 130.
Referring to FIG. 2, there is shown another example of a prior art OPU that provides BD/HD-DVD read/write access and legacy CD/DVD backward compatibility. The OPU 200 includes a three-wavelength laser diode 210 (i.e., three SC lasers that are co-packaged with very small lateral offset between the light emitting junctions), a first cube polarization beam-splitter (PBS) 231 for separating the write-beam and the read-beam into two orthogonal paths, a second cube dichroic beam-splitter (DBS) 232 for further separating the read-beam into a first path to the BD/HD-DVD disc photodiode 271 and a second path to the CD/DVD legacy disc photodiode 272 (i.e, a long-wave pass (LWP) filter immersed in glass media, transmitting the long CD/DVD wavelengths and reflecting the short BD/HD-DVD wavelength). The write beam passes through a collimating lens system 260, a 45 degree prism 240, and an objective lens 261, before reaching the disc media 262. The remaining components include a diffraction grating and various lenses, as is well known in the art. A QWP 245 is inserted in between the collimating lens system 260 and the objective lens 261. As discussed above, the fast/slow axes of the QWP are aligned approximately ±45 degrees with respect to the system ‘S’ and ‘P’ axes so as to provide circularly polarized light upon first pass there through.
In each of the OPU systems illustrated in FIGS. 1 and 2, the QWP functions as a polarization converter by, in a first pass, transforming linearly polarized light having a first polarization state to circularly polarized light, and in a second pass, transforming circularly polarized light into linear polarized light having a second orthogonal polarization state. Conventionally, QWPs are formed from birefringent elements such as inorganic crystals (e.g., single crystal quartz, single crystal MgF2, LiNbO3, etc.), liquid crystals, or stretched polymer films (e.g., polycarbonate, polyvinyl alcohol, etc). Unfortunately, conventional QWPs only function efficiently within a small wavelength band.
Accordingly, OPU systems, such as those illustrated in FIGS. 1 and 2, often use an achromatic QWP (AQWP), which provides quarter-wave retardance at more than one wavelength band and/or over a relatively broad wavelength band. Conventionally, AQWPs are fabricated by laminating two or more different waveplates together (e.g., a half-waveplate layer and a quarter-waveplate layer of two different index dispersion birefringent materials, such as quartz and MgF2, bonded together with an adhesive with their optical axes orthogonal to one another, or two or more layers of similar birefringent layers aligned with predetermined azimuthal angle offsets). However, while laminated AQWP structures do provide an increased bandwidth, they are also associated with poor environmental resistance. In addition, the use of two or more waveplate layers increases manufacturing costs of the AQWPs due to the required thickness and azimuthal angle offset tolerances.
With the current high density optical storage systems (i.e., one that includes a HD-DVD or BD disc reading/writing channel), the reliability of the QWP element becomes a critical factor at high power blue-violet laser output (e.g., 240 mW or higher power for faster read/write speed). Furthermore, an AQWP for all three light channels, blue-violet 405 nm, red 660 nm and NIR 780 nm is required to produce approximately, 100 nm, 165 nm and 200 nm of retardation magnitudes. These disparate retardation magnitude requirements, obtained from a high reliability birefringent component and at a low cost for consumer electronic integration, drive the search of alternate QWP technology other than single crystalline materials and stretched organic foils. One solution might involve separating the short wavelength blue-violet channel with its own OPU and the legacy red/NIR DVD/CD channels with a conventional OPU, including a stretched foil AQWP. However, this approach increases costs since there are multiple redundant optical components, fold mirrors, lenses, etc.
It is well known in the industry that an optical thin film having a series of homogeneous, isotropic dielectric layers and fabricated by high-vacuum deposition processes yields different phase changes upon reflection and transmission for a linear polarization aligned parallel to the plane of incidence (P-pol.) and perpendicular to the plane of incidence (S-pol). The fundamental reason for the phase changes in reflection and transmission at non-normal incidence is the effective index of refraction changes for P-pol. and S-pol. as a function of angles of incidence:
            n      p        =                            n                      cos            ⁢                                                  ⁢                          (              θ              )                                      ⁢                                  ⁢        and        ⁢                                  ⁢                  n          s                    =              n        ⁢                                  ⁢                  cos          ⁡                      (            θ            )                                ,where np and ns are the effective indices at θ angle of refraction from layer normal, θ is related to the angle of incidence θ0 by Snell's law,n0 sin(θ0)=n sin(θ),where n0 is the refractive index of incidence medium and n is the refractive index of a homogeneous thin film layer.
Given this historical knowledge, a thin film stack for transmissive or reflective operation can be designed, where retardation performance at non-normal incidence in addition to the filter power characteristics (such as short-wave pass, band-pass, anti-reflection, high reflection, etc) are achieved. One such design example is found U.S. Pat. No. 4,312,570 to Southwell, which teaches the design of a QWP (i.e., 90° retarder) at 45-deg. angle of incidence, utilizing a series of less than quarter-wave optical thickness (QWOT) layers at 10.6 μm wavelength. In this design, the stack of film is essentially transparent and the high reflectance is substantially obtained by the underlying silver substrate. In addition, this design is inherently narrow band (i.e., small fractions of useful wavelengths relative to the design center wavelength where the power and retardation properties can be achieved). Another design example is found in U.S. Pat. No. 5,196,953 to Yeh et al, which teaches form-birefringence using a series of alternative index thin layers to provide for net retardation at angles over a large bandwidth. Yet another design example is provided in U.S. patent Ser. No. 11/753,946, filed May 25, 2007, which is hereby incorporated by reference. With these homogeneous dielectric thin film coatings, one can only realize what is termed C-plate birefringent symmetry. The stack of thin film is either a positive or a negative C-plate, with is C-axis (the optic axis of an effective uniaxial indicatrix) aligned parallel to substrate normal.
If one examines the layout of a conventional OPU system, there is natural beam folding location, where the light beam traversing a series of optical components, all populating a given plane (e.g., horizontal plane), is steered through a 90 degree direction change in order to access the disc media. The folding optic is typically a 45 degree inclined high-reflector plate or a triangular prism with the inclined surface coated with a high reflector film.
The use of a thin film AQWP, in place of a standalone conventional AQWP in an OPU system is attractive for several reasons. The thin film AQWP can be made of high reliability dielectric layers; it does not involve growing and polishing birefringent crystal plates, hence it can be made at a lower cost; and it is not subjected to photo-chemical degradation of blue-violet lasers as in stretched polymer foils. For these reasons, there has been increased interest in replacing the conventional AQWP with an inclined thin-film coated plate having a phase shift property.
In US Pat. Appl. No. 2004/0246876, Kim et al. show a 2-wavelength DVD/CD OPU that uses a phase shift coating layer to create a 90 degree phase delay between P- and S-polarized light. The phase shift coating layer, which they state can be formed on any number of components, is intended to replace the conventional achromatic quarter-wave plate, thus providing a reduction in manufacturing costs. In one embodiment, the phase shift coating is formed on the fold mirror such that light is incident thereon at a 45 degree angle. In this embodiment, one skilled in the art would expect the phase shift coating to function as a C-plate. Unfortunately, the proposed OPU layout does not account for the fact that the slow and fast axes of the inclined C-plate are confined parallel and orthogonal to its tilt axis. Accordingly, the proposed OPU design, while incorporating a phase shift coating, does not convert linear polarized light input to left or right hand circularly polarized output and vice versa upon reflection off the fold mirror. Without this retardation effect, there is no change in polarization of the first pass incident light beam and the second return light beam, such that all light beams from both DVD/CD laser emitters are returned to the laser sources instead of being steered to the detector via a polarization beamsplitter.
In US Pat. Appl. No. 2006/0126459, Moon et al. show a 2-wavelength DVD/CD OPU that uses a phase shift mirror that also includes a coating corresponding to a quarter-wave plate on its surface. In contrast to the design proposed in US Pat. Appl. No. 2004/0246876, this configuration addresses the fact that the linearly polarized light must be incident on the phase shift mirror with its direction of polarization tilted at a predetermined angle (e.g., 45 degrees) and thus accounts for the fact that the slow and fast axes of the C-plate are confined to its tilt axis. Unfortunately, while this design does provide an improvement, it is lacking in that it is lossy, relatively complex, and limited to only two wavelengths. With regard to the former, most of the loss appears to originate from the use of a plate beam splitter, which separates all of the laser source optics, which are located on one side of the plate beam-splitter, from the detector, which is located on a second other side of the plate beam-splitter. The plate beam splitter is angled relative to cubic beam splitter such that both S-pol. and P-pol. light beams relative to the cubic beam splitter are incident on the plate beam-splitter as approx. half S-pol. and half P-pol. in the first pass from the laser sources to the disc media; and such that the plate beam splitter functions as a non-polarizing beam splitter (e.g., 50:50 intensity split). This intensity split, which is encountered twice, results in significant light loss.
In US Pat. Appl. No. 2006/0039265, Lee also discloses an OPU that does not use a conventional, standalone AQWP. More specifically, Lee discloses the use of multiple optical thin films, along the light path between the laser diode and the disc media, including polarization beam-splitter (PBS) cubes and fold mirror reflector plate to generate the cumulative 90 degree S-pol. vs. P-pol. phase difference. Unfortunately, the design, which generally includes at least one PBS cube, does not take the diattenuation effects of the PBS cube(s) into account. With diattenuation, which was stated to be roughly 90% (i.e., 0% S-pol. reflected and 90% P-pol. reflected), the phase change of the attenuated linear polarization direction does not matter. There is no reflected light in the attenuated direction to cause birefringent effects. Consequently, the output of the PBS cubes is strictly P-pol. or S-pol. Notably, Lee also fails to provide an azimuthal offset of the incident linear polarization with respect to the partial QWP phase coating on the fold mirror. Accordingly, the light reflected from the fold mirror will not be changed from linear to circular polarization. The return light from the disc media will be steered towards the laser sources, instead of the detector owing to the unchanged polarization.
It is an object of the instant invention to provide an OPU incorporating a coating that provides substantially quarter-wave retardance (i.e., a 90 degree phase shift) with a configuration that obviates at least some of the above-described problems of the prior art.